In the past ten years, there has been a surge in the development of materials suitable for withstanding high temperatures, particularly those that are encountered in space reentry vehicles, rocket nozzles, turbines, internal combustion engines and the like. Materials used for such applications should exhibit high-temperature strength and resistance to thermal shock, as well as the ability to resist abrasion. Two key properties that must be displayed by such materials are (1) resistance to chemical degradation, particularly oxidation and/or reduction at such temperatures, and (2) sufficient thermal conductivity to diffuse thermal stress. Materials that have considerable potential for high-temperature applications are ceramic or metal matrix composites. To date, however, neither type of composite has been developed with significant mechanical strength at temperatures much above 1200.degree. C.-1400.degree. C.
Considerable effort is presently being expended to extend the useful range of ceramic or metal matrix composites. One approach has been to reinforce composites with fibrous material. Light-weight carbonaceous fibers are particularly attractive because they can be fabricated in a wide variety of tensile strengths and moduli of elasticity. Moreover, the thermal conductivity of these fibers is eight times that of copper and the fibers also exhibit a slightly negative coefficient of thermal expansion.
While carbonaceous fibers composed of carbon, carbides, or graphite have proven to increase the useful mechanical properties of resin matrix composites and some metal matrix composites, they nevertheless do not confer stability much beyond 1500.degree. C. In part, this is because the matrices of such composites readily degrade carbonaceous fibers during composite fabricating. Fabrication must often be performed at significantly higher temperatures than those compatible with the otherwise useful carbonaceous fibers. Thus carbonaceous fibers are of limited use in high-temperature applications because of the tendency of these fibers to deteriorate. For instance, graphite fibers can withstand temperatures approaching 2200.degree. C. in vacuum but oxidize readily in the air above 316.degree. C. and unprotected graphite fibers at temperatures above 1350.degree. C.-1500.degree. C. in a SiAlON matrix are readily destroyed during composite fabrication.
Attempts to prevent oxidation of carbonaceous fibers have met with limited success. One approach has been to coat carbon fibers with silicon carbide, which in turn provides a secondary layer of silicon dioxide as a shield over the silicon carbide primary coating. This procedure works well at temperatures below 1200.degree. C. Above this temperature, however, silicon dioxide undergoes a phase change and there is a loss of protection for the underlying carbonaceous fiber.
As noted above, carbonaceous fibers display superior thermal conductivity properties. Ceramics reinforced with carbonaceous fibers enjoy improved resistance to thermomechanical shock because the fibers conduct heat away from the site of impact. However, at high temperatures carbonaceous fibers are oxidized or reduced, and consequently fiber reinforced ceramic composites in which they are incorporated undergo premature catastrophic failure. Thus, in order to take further advantage of carbonaceous fibers in fabricating fiber reinforced ceramic or metal matrix composites, it is desirable that methods of shielding the fibers from chemical degradation at high temperatures be developed.
U.S. Pat. No. 4,376,803 describes a method for coating carbon fibers that can be used in metal matrix composites. The coated carbon fibers are not useful at high temperatures because they are (1) coated with metal oxides having low melting temperatures, such as oxides of silicon, titanium, vanadium, lithium, sodium, potassium, zirconium or boron or (2) coated with a high melting temperature oxide such as magnesium oxide that is applied by a method that forms a porous oxide coating about the fibers. Fibers produced by the latter method are not suitable for high temperature applications because oxygen diffuses through the pores resulting in oxidation of the fibers.
Considering the enhanced properties that ceramic or metal composite matrices reinforced with carbonaceous fibers display, it is particularly desirable to develop a process for protecting such fibers so that they are resistant to chemical destruction at high temperatures.